Re: Using debug to find the enclosing function

[Roberto Ierusalimschy <roberto@...>]

> Consider the following:
> 
> function foo ()
>   function bar () end
> end
> 
> Is there a precise way, at runtime, to find the function that encloses bar?
> 
> It appears the best (but imprecise) approximation is to use
> debug.getinfo() and then compare the values of linedefined and
> lastlinedefined for each function.
> 
> (This approximation would be incorrect (or inconclusive) in the
> (possibly rare) circumstance where two functions were completely
> defined on the same line(s).)

Why do you need this information?

In Lua, functions are not a thing. What is a thing are closures,
dynamically created instances of function prototypes.  So, a program can
have several instances of "foo" and several instances of "bar", or even
some instances of "bar" without any instance of "foo".

In particular, the notion of "the function A that encloses another
function B" is actually between closures and, in particular, A
may not necessarily exist during all existence of B:

  function foo ()
    function bar () end
  end
  -- foo refers to 1 instance of "foo", there are no "bar"s

  foo()
  -- foo refers to 1 instance of "foo" and bar to 1 instance of "bar"

  foo = nil
  -- bar refers to 1 instance of "bar", there are no "foo"s

-- Roberto